newlib/libc/stdlib/realloc.c: fix variable name

The variable doesn't follow the convention of having the same name as
the function it's bundled with. Furthermore, it clashes with the
variable of the same name in newlib/libc/stdlib/calloc.c.

Signed-off-by: Florian Schmidt <florian.schmidt@neclab.eu>

newlib/libc/stdlib/realloc.c

@@ -1,5 +1,5 @@
 #ifdef MALLOC_PROVIDED
-int _dummy_calloc = 1;
+int _dummy_realloc = 1;
 #else
 /* realloc.c -- a wrapper for realloc_r.  */